1. Field of the Invention
The present invention relates to a dynamo-electric machine and a vehicular air blower having the same.
2. Description of Related Art
For example, as recited in Japanese Unexamined Patent Publication No. 2005-29038 (corresponding to U.S. Pat. No. 7,067,946 B2 and U.S. 2006/0192449 A1), there is known a vehicular air blower, which includes a fan rotated by a direct current motor to blow air into a passenger compartment of a vehicle. In an armature of the direct current motor of the above vehicular air blower, electric current is supplied to the armature from brushes, which are slidably engaged with a commutator that is fixed to a rotatable shaft of the armature. When the electric current is supplied to the armature through the brushes, the fan, which is fixed to the rotatable shaft, is rotated together with the rotatable shaft.
In the above direct current motor, at the time of supplying the electric current to the armature, when the rotation of the rotatable shaft is locked or when an excess load is applied to the rotatable shaft, excessive electric current is supplied to the armature through the brushes. In such a case, the brushes may generate abnormal heat due to the excess electric current to cause burnout of the direct current motor. Thus, in order to limit the burnout caused by the abnormal heat generation of the brushes, a protective device, such as a fuse, is provided in the direct current motor or on an upstream side of an electric power source of the direction current motor, so that the supply of the electric current to the armature is stopped whenever the excessive electric current is supplied to the armature.
In a case where the fuse malfunctions due to some reason, the excessive electric current may possibly be supplied to the armature through the brushes. Thus, in general, in addition to the fuse, the direct current motor further includes a safety device, which stops the supply of electric current from the brushes to the commutator when the excessive electric current is supplied to the armature. One such safety device includes a circuit. In this circuit, the electric current, which is supplied to the armature, is measured, and the measured electric current is compared with a predetermined threshold value. When the measured electric current is larger than the threshold value, the circuit stops the supply of electric current to the armature. Furthermore, in order to reduce occurrence of the damage caused by the burnout, the resin components of the direct current motor are made of flame-retarded resin.
When the safety device is provided to the direct current motor in addition to the fuse, a size of the direct current motor becomes large, or a structure of the direct current motor becomes complicated. Furthermore, the flame-retarded resin is generally expensive. Thus, when such expensive flame-retarded resin is used, the manufacturing cost of the direct current motor is disadvantageously increased. The above disadvantages are not limited to the direct current motor of the vehicular air blower and are common to dynamo-electric machines, which are energized through brushes.